The biggest issue for me is the fact that I still play SHIII, and will for some time yet, it is now 8 years old, will UBI servers still be active for SH5 in 8 or more years?

I think not, when they drop support for the game they will cut us off, when it happens the game can no longer be played, and who would want to mod a game knowing it could be dropped any time
they feel like it, not me and others feel the same way I'm sure!

Basically your leasing the game till your lease runs out, that will be at UBI's discretion!